General query

[nicnac]

I have noticed on various threads people writing that with our conditions in the UK magnifications upwards and beyond 300x are rarely feasible/attainable/usable.

So, my question is why bother acquiring a scope with capabilities higher than 300x ??

[John]

Good question !.

Personally I like using my scopes at less than their theoretical max power (much less in the case of my 12" dobsonian). As the seeing conditions come and go you get glimpses of the extra details that the larger aperture can show wheras if the scope was "flat out" so to speak it would have nothing more to give in those short periods excellent seeing - it would have no "11" as Nigel Tufnel would say ....

John

[James]

Once, just once, whilst at Kelling Heath at the April 2007 spring star party there was a clear night with such good seeing that I was able to view Saturn at 400x in my 10in reflector. Conditions have never been like that since but the view was stunning and it wouldn't have happened if I didn't have the necessary eyepiece and barlow - they paid for themselves that night.

That said I second John above - generally I prefer to observe at magnifications well below my scopes maximum capability.

James

[Sunwind]

I always thought that the larger the scope the more light you could catch, so you could see things better, regardless of how much magnification you used, so even if you use magnifications much lower than your scope is capable of, you will still be seeing better than someone with a smaller scope that isn't catching as much light, right?

[John]

I always thought that the larger the scope the more light you could catch, so you could see things better, regardless of how much magnification you used, so even if you use magnifications much lower than your scope is capable of, you will still be seeing better than someone with a smaller scope that isn't catching as much light, right?

Yep, thats my understanding as well

I always think of a scope as a light collecting instrument. It's the eyepiece that magnifies the image - two optical instruments working together I guess.

John

[iamjulian]

I have a very modest 6" scope with 5 eye pieces and a barlow which allow me the following magnifications: 24x, 30x, 60x, 75x, 83x, 125x, 150x, 166x, 250x.

I spend at least 75% of my time at 24x magnification. Partly because it is the best quality eye piece I have, partly because I look at stars and galaxies much more than planets, but mostly because less magnification gives a wider field of view which for me is more pleasing than looking through a dark keyhole.

[acey]

There are two issues - brightness and sharpness. Both are related to aperture.

A larger lens or mirror will gather more light (resulting in brighter images) and will also have greater resolution (sharper images). The latter theoretically means that the more aperture you have, the higher the magnification you can reach - with the atmosphere setting the final limit.

But resolution varies linearly with aperture while brightness varies as the square. And the weight and bulk of a scope goes roughly as the cubic power. And there are other issues that distinguish refractors and reflectors in terms of resolution and contrast.

The upshot is that if you want to look at planets then a top-notch 4-inch scope may be all you need. If you want to look at faint galaxies then the more aperture you can manage, the better.

My 12-inch scope gives planetary views no better than my 8-inch, but on deep sky it's no contest. For the deep sky observer, aperture is all that matters. For the planetary observer it's more complicated.

My highest power eyepiece gives x200 on the 8-inch, x250 on the 12-inch. That's high enough for me. If I ever get a bigger scope it won't be because I want higher magnification.

[brianb]

I always thought that the larger the scope the more light you could catch, so you could see things better, regardless of how much magnification you used, so even if you use magnifications much lower than your scope is capable of, you will still be seeing better than someone with a smaller scope that isn't catching as much light, right?

Precisely. Aperture rules.

You could argue that (for visual work) it wouldn't be worth having a scope with a minimum useful magnification exceeding 300x, but since the aperture of such a scope would be 60", it's really a moot point.

[acey]

I don't understand the preceding post.

The usual rule of thumb is 50x aperture in inches. To have a highest usable magnification of 300x you need a scope of 6" aperture. The issue is why you might ever need more.

On some (rare) nights it's possible to use magnifications of 600 or more. At such times the resolving power of a 12" scope can be tested to the full. Its light-gathering power can be appreciated on any clear night.

[johninderby]

A lot also depends on the quality of the optics as well as the aperture, particularly with a reflecting scope such as a dob. A top quality premium mirror with it's very smooth finish doesn't scatter light as much as a mass produced mirror and lets you use higher magnifications and still have a sharp and contrasty image.

With my 10" Skywatcher dob 250 mag is about the max for general observing and up to 400 on lunar. With my 14" dob with it's premium 99 strehl mirror on a night of decent seeing 500 mag is OK on Jupiter for example with up to 800 on lunar. Of course on nights of exceptional seeing it can do even better.

John

[pvaz]

I don't understand the preceding post.

The usual rule of thumb is 50x aperture in inches. To have a highest usable magnification of 300x you need a scope of 6" aperture. The issue is why you might ever need more.

On some (rare) nights it's possible to use magnifications of 600 or more. At such times the resolving power of a 12" scope can be tested to the full. Its light-gathering power can be appreciated on any clear night.

Brianb was referring to the MINIMAL useful aperture. As you increase aperture you need very precise optics on the edges or the coma and other problems will be in unacceptable values. To avoid this need for precision, the usual solution is to increase the focal length witch increases the theoretical minimum useful magnification.

[John]

.... With my 14" dob with it's premium 99 strehl mirror on a night of decent seeing 500 mag is OK on Jupiter ......

Now THAT would be a view worth seeing John

John

[Doc]

In my case I could get x800 out of my 16" if I use the formula X50 for every inch of aperture but with the seeing we get in the UK thats never going to happen.

My 7 Uwan gives me x261 which turns out to be pretty steady while viewing planets. The 4 Uwan gives x457 which is rarely used as it's just to much power, but does do well on the moon.

[NickH]

Regularly imaged the sun in H-alpha, tack sharp at 533X magnification, have twice had Saturn with a 4mm EP on the C11 crystal clear, but yes, it's rare for night time as hens teeth. The rule of thumb is more for visual in most cases

[barkis]

Surely A larger area objective collect more gunge as well as the good stuff, so a smaller one may have an advantage on a planet for example, bad seeing neutralises the potential higher magnification of the bigger beast. But, I will still take the big one, and wait in vain for the good night.

Ron.

[great_bear]

bad seeing neutralises the potential higher magnification of the bigger beast

I wouldn't know personally - but there is a school of thought that the "big scope multiplies bad seeing" phenomenon is overplayed, and the real culprit is scope currents - which can be fixed.

There's certainly a lot to think about in the refractors vs. reflectors thread on Cloudy Nights

[brianb]

but there is a school of thought that the "big scope multiplies bad seeing" phenomenon is overplayed

That's my experience. I've NEVER been able to see MORE with a very high quality 110mm refractor than with a 8" or 11" SCT located at the same site, though when the seeing is really bad the 'frac shows as much. Visually. For imaging, extra light is always a help (allowing shorter exposures which cut through bad seeing), and the very slight loss of contrast caused by the central obstruction can easily be processed out.